1. Field
The present invention relates to inductors, and, more particularly, to structures and methods for generating an inductance in a circuit.
2. Description of the Related Art
Classical inductors typically include an input, an output and a coil disposed therebetween through which current rotates. The width of each coil turn is typically equal to the thickness of the coil turn. In an integrated circuit, the width of each coil turn is typically equal to the thickness of the available metal layers. There has been an increasing need to design a small inductor with a good quality factor (Q). This need is especially pronounced in high frequency voltage controlled oscillator (VCO) designs. However, such inductors have been very difficult to design given modern integrated circuit design restrictions. Traditionally, inductors are designed to be metal traces forming various loops. One limitation on this type of inductor is that as the inductance value L decreases, its quality factor Q tends to get smaller as well. This makes small inductors less useful in high-frequency, low-loss VCO designs.
One resolution of this problem is to place multiple inductors in parallel to reduce the effective inductance. For example, when two identical inductors are placed in parallel, the effective inductance Leff of the parallel inductors is the combined inductance Lc of each inductor plus the mutual inductance M between the inductors divided by two.
      L    eff    =                    L        c            +      M        2  If the parallel inductors are placed too close together, M is substantially equal to Lc (M≈Lc), and there is little or no inductance reduction at all. If the inductors are placed sufficiently far away from each other, M≈0. In this case,
      L    eff    =                    L        c            2        .  However, sufficient isolation is difficult to obtain. Also, there is a limit on the number of inductors that can be placed together in parallel given the fact that their mutual inductances need to be small to get effective inductance reduction. Thus, there is a need for a new inductor and method for generating an inductance, e.g., to generate a small inductance L with sufficient quality factor Q.